As disclosed in the above identified parent application, solar hydrogen generation by photovoltaic-electrolyzer (PV-electrolyzer) systems is a renewable and environmentally beneficial energy source for fuel cell vehicles and other applications utilizing hydrogen as a fuel. But the photovoltaic system and the hydrogen-producing electrolyzer are separate and distinct operating devices whose usage and operations must be coordinated to achieve suitable operating efficiencies for each when they are used in combination.
A photovoltaic system typically comprises a group of individual planar solar cells arranged in rows and columns in a flat panel called a module. Each cell in a module is typically made of the same chemical material having the property of converting incident solar radiation to an electrical potential. Materials for such photovoltaic cells include, for example, crystalline silicon, amorphous silicon, copper-indium-selenium (CuInSe2), or cadmium-tellurium (CdTe). A representative cell membrane might, for example, produce an open-circuit, direct current electrical potential of 0.6 V at a cell membrane temperature of 25° C. when receiving solar radiation of 100 mW/cm2 (one sun irradiance). The several cells in a planar module may be arranged and electrically connected to produce a specified operating voltage and direct current at a specified temperature and under specified sun irradiance and operating load conditions. Two or more modules may be connected in series or parallel electrical connection in a group of modules called an array.
There are also known electrolyzer systems for the electrolytic dissociation of water into hydrogen and oxygen. Examples include alkaline electrolyzers, proton exchange membrane (PEM) electrolyzers, steam electrolyzers and high pressure electrolyzers. For many applications an alkaline electrolyzer may be preferred. The electrolyzer typically consists of a group of individual cells that are interconnected electrically to obtain a desired rate of hydrogen production using specified electrical power parameters. The individual alkaline water electrolyzer may, for example, comprise an aqueous potassium hydroxide (5M KOH) electrolyte, a platinum or nickel cathode (for hydrogen) and a suitably catalyzed anode for oxygen generation.
In the design of a specific hydrogen generation operation the electrolyzer is designed and specified for a desired hydrogen production rate. The electrolyzer design will have specified number of electrolyzer cells at a DC voltage/cell of about 1.6 volts and an electrical power requirement for the scheduled hydrogen production rate and operating temperature range of the system. The several electrolytic cells may be arranged in series or parallel electrical connection. A photovoltaic system is then provided with the capability of efficiently delivering electrical power to the electrolyzer.
It has been recognized that a given PV system of cells and modules has a maximum power point voltage for the system that is found from a predetermined relationship between an actual voltage and actual current under load. It is recognized that improved efficiencies are gained by modifying the number of electrolyzer cells so that a PV system can be operated at its maximum power point voltage. Conversely, the number of modules in the PV system can be varied so that the load required by the electrolyzer matches the revised the reconfigured PV system. However, the operations of the PV system and electrolyzer system can vary. For example, the operation of the PV system is particularly subject to variation in ambient temperature and solar irradiance. In this example, there is a continual need to recognize changing operating characteristics of the PV system and adapt the overall operation of the PV-electrolyzer to such changes in order to maintain operating efficiencies of the combined systems.
Accordingly, there remains a need for practices for optimizing the operation of a group of photovoltaic modules (arrays) in combination with an electrolyzer with a group of cells for the electrolysis of water into hydrogen and oxygen.